"This research highlights that breast cancer patients who smoke need to be offered help and support in order to try and quit to minimize any risks from their treatment," Dr. Julie Sharp said in a Cancer Research UK news release. She's head of health information for the research and awareness charity based in the United Kingdom.

"It's important to remember that modern-day radiotherapy techniques have been refined and improved to make sure it is targeted and effective while reducing the risk of side effects," Sharp added.

British researchers looked at data from nearly 41,000 breast cancer patients. The information came from 75 different studies on radiation therapy for breast cancer.

The researchers found that the long-term risk of heart attack or lung cancer -- caused by radiation -- was about 5 percent in smokers, compared with 0.5 percent in nonsmokers.

Study lead author Dr. Carolyn Taylor is from the University of Oxford. "Stopping smoking at the time of radiotherapy will avoid most of the lung cancer and heart disease risk from radiotherapy, and has many other benefits," she said.